5 Things to Consider When Thinking of a Dormer Addition

Bright bathroom with glass shower, cream vanity, and large window with blinds

This dormer addition, designed by Arciform designer Chelly Wentworth, stayed under the 12′ size restrictions.

There are many things to think about when designing a dormer addition. Here are 5 things your designer will take into consideration.

1. Size

The size of your dormer addition can determine whether you will need additional permits and upgrades. By keeping it under 12′ you can typically avoid major seismic upgrades like sheer wall upgrades or adding posts and footings.

Architectural elevation drawing of a two-story house with windows, siding, and front door.

2. Stacking

Stacking a bath over an existing is the most cost effective way to add a bathroom to a dormer. By tapping into the existing plumbing from below you will avoid major plumbing costs.

To avoid sizable seismic and structural challenges, stack new windows over existing ones. By doing so, the load of your house will not change significantly.

Attic HVAC ductwork and white PVC pipe between wooden joists, with metal vents and insulation

3. Joist Depth

The depth of existing joists can play a large role in whether or not your dormer addition will allow for a bathroom. The joists must be a certain depth to accommodate plumbing lines, specifically for a toilet. Converting an attic into a dormer may also require structural improvements for the purpose of resisting wind or earthquake load and floor live loads (people, furniture).

Architectural section drawing of a building interior with stairs, doors, and structural lines

A plan by Arciform designer Chelly Wentworth shows the insulation (pink area) in a dormer addition.

4. Insulation

When building a dormer the required roof insulation is rated at R-38, which requires a minimum 2×8 rafter size. If you are planning to install recessed can lighting fixtures you will need a 2×10 rafter to meet insulation requirements. This can provide challenges when you are dealing with existing framing members that are smaller and if you are trying to keep a continuous roof or wall plane.

Backyard view of a tan house with white patio chairs, sliding door, and green lawn

This main floor addition by Arciform designer Kristyn Bester fits seamlessly with the aesthetics of the house.

5. Aesthetics

Make sure the type of dormer you are selecting is appropriate for your house – will it work with the existing roof lines? The two most common dormers are sheds and gable.

Exterior of a gray house with a sloped roof and upper-row windows, surrounded by trees

A shed dormer designed by Arciform designer Anne De Wolf that ties into the existing roof lines.

Shed Dormer:

Typically off the back of the house, has a flat shed-like roof and allows for more headroom. These aren’t always aesthetically pleasing so adding character to the side will help to keep it appealing.